MONSOON IN INDIA

• There are variations in weather conditions during different seasons.  These changes occur due to the changes in the elements of weather (temperature, pressure, wind direction and velocity, humidity and precipitation, etc.).
• Weather is the momentary state of the atmosphere while climate refers to the average of the weather conditions over a longer period of time.
• Monsoon connotes the climate associated with seasonal reversal in the direction of winds.
• India has hot monsoonal climate which is the prevalent climate in south and southeast Asia.
• Monsoon regime emphasizes the unity of India with the rest of Southeast Asian region.
• The climate of India has many regional variations expressed in the pattern of winds, temperature and rainfall, rhythm of seasons and the degree of wetness or dryness. These regional diversities may be described as sub-types of monsoon climate.
• There are seasonal variations in temperature from place to place and from region to region in India.
• Most parts of the country get rainfall during June-September, but on the coastal areas of Tamil Nadu, it rains in the beginning of the winter season.

FACTORS DETERMINING THE CLIMATE OF INDIA

Factors related to Location and Relief

Latitude 

• Tropic of Cancer passes through the central part of India in east-west direction. Thus, northern part of the India lies in sub-tropical and temperate zone and the part lying south of the Tropic of Cancer falls in the tropical zone.

The Himalayan Mountains

• The lofty Himalayas in the north along with its extensions act as an effective climatic divide.
• The towering mountain chain provides an invincible shield to protect the subcontinent from the cold northern winds.
• These cold and chilly winds originate near the Arctic circle and blow across central and eastern Asia.
• The Himalayas also trap the monsoon winds, forcing them to shed their moisture within the subcontinent.

Distribution of Land and Water

• Indian Ocean on three sides in the south and girdled by a high and continuous mountain-wall in the north.
• This differential heating of land and sea creates different air pressure zones in different seasons in and around the Indian subcontinent.
• Difference in air pressure causes reversal in the direction of monsoon winds.

Distance from the Sea

• Areas in the interior of India are far away from the moderating influence of the sea. Such areas have extremes of climate

Altitude

• Temperature decreases with height.
• Due to thin air, places in the mountains are cooler than places on the plains.

Relief

• Physiography or relief of India also affects the temperature, air pressure, direction and speed of wind and the amount and distribution of rainfall e.g. windward sides of Western Ghats and Assam receive high rainfall during June-September whereas the southern plateau remains dry due to its leeward situation along the Western Ghats.

Factors Related to Air Pressure and Wind

1.  Distribution of air pressure and winds on the surface of the earth.
2.  Upper air circulation caused by factors controlling global weather and the inflow of different air masses and jet streams.
3.  Inflow of western cyclones generally known as disturbances during the winter season.
4. Tropical depressions during the south-west monsoon period into India, creating weather conditions favorable to rainfall

Mechanism of Weather in the Winter Season

Surface Pressure and Winds

•  In winter months, the weather conditions over India are generally influenced by the distribution of pressure in Central and Western Asia.
• The surface winds blowing out of the high pressure center over Central Asia reach India in the form of a dry continental air mass.
• These continental winds come in contact with trade winds over northwestern India.
• Result – whole of the northwestern and northern India up to the middle Ganga valley comes under the influence of dry northwestern winds.

Jet Stream and Upper Air Circulation

• Higher up in the lower troposphere, about 3 km above the surface of the earth, a different pattern of air circulation is observed.
• The variations in the atmospheric pressure closer to the surface of the earth have no role to play in the making of upper air circulation.
• All of Western and Central Asia remains under the influence of westerly winds along the altitude of 9-13 km from west to east.
• These winds blow across the Asian continent at latitudes north of the Himalayas roughly parallel to the Tibetan highlands. These are known as jet streams
• Tibetan highlands act as a barrier in the path of these jet streams. As a result, jet streams get bifurcated.
• One of its branches blows to the north of the Tibetan highlands, while the southern branch blows in an eastward direction, south of the Himalayas (February mean position at 25°N)
• Southern branch of the jet stream exercises an important influence on the winter weather in India.

Western Cyclonic Disturbance and Tropical Cyclones

• Western cyclonic disturbances which enter the Indian subcontinent from the west and the northwest during the winter months originate over the Mediterranean Sea and are brought into India by the westerly jet stream.
• An increase in the prevailing night temperature generally indicates an advance in the arrival of these cyclones disturbances.
• Tropical cyclones originate over the Bay of Bengal and the Indian Ocean.
• These tropical cyclones have very high wind velocity and heavy rainfall and hit the Tamil Nadu, Andhra Pradesh and Orissa coast.
• Very destructive due to high wind velocity and torrential rain that accompanies it.

Mechanism of Weather in the Summer Season

Surface Pressure and Winds

• As the summer sets in and the sun shifts northwards, the wind circulation over the subcontinent undergoes a complete reversal at both, the lower as well as the upper levels.
• By the middle of July, the low pressure belt nearer the surface [termed as Inter Tropical Convergence Zone (ITCZ)] shifts northwards, roughly parallel to the Himalayas between 20° N and 25° N.
• By this time, the westerly jet stream withdraws from the Indian region.
• ITCZ and Westerly Jet Stream
• Found interrelationship between the northward shift of the equatorial trough (ITCZ) and the withdrawal of the westerly jet stream from over the North Indian Plain. It is generally believed that there is a cause and effect relationship between the two.

Southwest monsoon

•  ITCZ being a zone of low pressure attracts inflow of winds from different directions.
• The maritime tropical airmass (mT) from the southern hemisphere, after crossing the equator, rushes to the low pressure area in the general southwesterly direction. It is this moist air current which is popularly known as the southwest monsoon

Jet Streams and Upper Air Circulation

•  The easterly jet stream sets in along 15°N latitude only after the western jet stream has withdrawn itself from the region. This easterly jet stream is held responsible for the burst of the monsoon in India.
• The easterlies normally do not extend to the north of 30o N latitude in the upper atmosphere.

Inter Tropical Convergence Zone (ITCZ)

• Inter Tropical Convergence Zone (ITCZ) is a low pressure zone located at the equator where trade winds converge, and so, it is a zone where air tends to ascend.
• In July, the ITCZ is located around 20°N-25°N latitudes (over the Gangetic plain), sometimes called the monsoon trough. This monsoon trough encourages the development of  thermal low over north and northwest India.
• Due to the shift of ITCZ, the trade winds of the southern hemisphere cross the equator between 40° and 60°E longitudes and start blowing from southwest to northeast due to the Coriolis force.
• It becomes southwest monsoon. In winter, the ITCZ moves southward, and so the reversal of winds from northeast to south and southwest, takes place. They are called northeast monsoons.